My duck died, what from?

My duck died but I don't know what got him, I saw him this morning when I was letting my chickens out, then I went to the grocery store, but when I came back I couldn't find him, I looked every were (he was free range) and I didn't find him or any feathers like something had taken him. I just want to know if this "thing" will come back and get my other duck or chickens.